Use of a sol–gel conversion coating for aluminum corrosion protection

By Zhang, Zheng; Zhe, Jiang; Chandra, Santanu & Hu, Jun
Published in Surface and Coatings Technology NULL 2001

Abstract

In this study, the behavior of a sol–gel conversion coating alone and in combination with a polyurethane unicoat (TT-P-2756, self-priming topcoat) on Al 2024-T3 alloy was investigated under immersion in dilute Harrison's solution [3.5 g/l (NH4)2SO4, 0.5 g/l NaCl]. The sol–gel coating consisted of SiO2 and ZrO2 with a ratio of 3.4:1. The evolution of the coating system under immersion was followed by atomic force microscopy (AFM), scanning electronic microscopy (SEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). It was found that though pitting corrosion and degradation products on the sol–gel single coating surface were observed after 2 days of immersion, further pitting corrosion ceased after a few days of immersion. Under-coating blisters in the sol–gel plus polyurethane topcoat system were found at 4 weeks of immersion, after which no further increase in size of the blisters was observed. It is conjectured that aluminum oxide and silicon oxide may form a stable mixed oxide barrier layer at the interface after initial corrosion, which prohibits further pitting corrosion development.

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